The present invention relates to improvements for boom hoist systems used in the operation of lifting cranes, and more particularly, to methods and apparatus for preventing a hoisted gantry from uncontrollably falling after hoist system failure.
Boom hoist systems are used to support and adjust the position of the spanning framework structure or gantry that provides mechanical advantage to lifting cranes. This is typically accomplished by a wire cable that attaches to a portion of a gantry at one end and couples to the drum of a hoist system at the other end. The weight of the gantry is supported by the wire cable attached to the hoist drum. When the gantry is maintained in a given position, the drum is locked in place by a brake to prevent the weight of the gantry from unwinding wire cable from the drum. If desired, the position of the gantry may be adjusted by rotating the hoist drum and altering the length of the wire cable supporting the gantry. Rotational power is supplied to the drum by a mechanical drive train that connects to the primary motor of the crane. When adjustment of the gantry is desired, the motor rotates the drum through the drive train causing the wire cable either to accumulate on or to dispense from the drum, thus raising or lowering the gantry so that the crane may be adjusted to a wide range of positions at varying degrees of leverage. In conventional systems, the weight of the gantry is dynamically supported through the drive train of the primary motor and held in place by a separate holding brake applied to the drive train near the motor.
One problem with this type of system is that a failure of the motor, holding brake, or drive train can cause a hoisted gantry to become unsupported and drop uncontrollably. This problem is further complicated by the sudden and unpredictable nature of such failures which leaves the crane operator with insufficient time to react to the problem. In the past, conventional systems have failed catastrophically causing complete destruction of the gantry and jeopardizing the safety of support personnel.
It would therefore be desirable to provide boom hoist systems with adequate security features to prevent such failures. It would also be desirable to provide a boom hoist system that is not dependent upon a mechanical drive train for support and rotational power. It would be additionally desirable to provide a boom hoist system with an independent fail-safe braking system to ensure continued support for a hoisted gantry in the event of a hoist system malfunction or failure.